This research protocol is a group-wide study which plans to treat children with newly diagnosed standard risk acute lymphoblastic leukemia (SR-ALL): ages 1-9 years, WBC <50,000 and <25% L3 blasts. This protocol asks two principal therapeutic questions for which patient randomization will occur in a 2X2 factorial design. The study will determine whether substitution of oral 6-Thioguanine (TG)for oral 6-mercaptopurine (MP) improves event free survival. The study will also determine whether treatment with "triple" intrathecal (IT) therapy (Methotrexate [MTX], cytarabine, hydrocortisone) rather than IT MTX alone reduces the incidence of central nervous system (CNS) relapse. The rationale for this question is based upon the low CNS relapse rates observed in Pediatric Oncology Group protocols that utilize "triple" IT therapy. Some patients will be non-randomly assigned to an alternate, more intensive regimen because of poor prognostic features identified during the first month of therapy. Another purpose of this study is to identify response-related factors predictive of relapse among patients with SR-ALL. Factors to be studied include the percent of residual marrow blasts at Day 7 and 14 of Induction, the percent of circulating blasts at the same time points, and the amount of minimal residual disease identified in marrow at three time points during treatment. Another specific aim is to determine whether CSF terminal deoxynucleotidyl transferase (TdT) positivity is predictive of CNS or BM relapse in children whose CSF contains low WBC and suspicious blasts during maintenance.